1. Field of the Invention
The present invention relates generally to a furnace process, and more particularly to a method for purging furnace to decrease particle pollution in baking process.
2. Description of the Prior Art
Conventional furnace process takes advantage of heat convection and heat conduction principles to make the silicon wafer to reach heat balance with circumstances around the furnace so the temperature on control wafer can be exactly controlled, and the furnace can heat a great quantity of wafers at a time. It is able to reach the objective that reduce process time and cost for batch process.
However, conventional furnace process has some defect of itself. Because the furnace wall need to be heated to reach high temperature condition for long so the impurities are easy to deposit in the furnace and to pollute the wafers so that conventional furnace process is difficult to apply in the deep sub-micro process. Therefore, the pending problem that is the residual impurities are produced during the furnace process requires to be solved. Conventional method is used to wash the furnace, but the purge material is remained during the purge process, that result in the particle problem. Hence, conventional furnace process is becoming more and more difficult to control the pollutant source when the dimension of device is shrink gradually.
Cross-sectional view of the known prior art of the furnace process is illustrated in FIG. 1A. First of all, a wafer 110 is placed into a furnace boat 100 to perform a baking process, wherein the furnace boat 100 includes an external tube 130 having a material of quartz and an inside tube 140 having a material of silicon carbide-silicon (SIC--Si). The cap 120 makes the wafer 110 to rise into the furnace boat 100 and proceed with the baking process at the high temperature and the low pressure.
Referring to FIG. 1B, an oxide layer 160 of impurities is formed on the wall of the inside tube 140 during the baking process, wherein the oxide layer 160 will increase thickness of itself to follow the process. It is necessary to perform a purge step to eliminate the oxide layer 160 which form on the wall of the inside tube 140 when the thickness of the oxide layer 160 increase to be standard value which is set in the apparatus, such as 400 K .ANG.. The purge step of above is performed by level-style washing using the water and hydrofluoric acid (HF), and using gas such as NH.sub.2 to dry the inside tube 140, then, the inside tube 140 which has been purged is set into furnace boat 100. For avoiding inside tube wall 150 to be damaged by oxide layer 160 at high temperature thermo-oxide process, hence it is necessary to perform a pre-coating process so as to form a silicon nitride layer (SiN) 170 over the inside tube wall 150 of furnace 100 in advance before the furnace 100 enter into the baking process of the wafer. The pre-coating process of above performs to transport a mixing gas which consists of SiH.sub.2 Cl.sub.2 (DCS) and ammonia (NH.sub.3) into the furnace 100 at the environment of high temperature and low pressure, and it proceeds to deposit a silicon nitride layer 170 on the inside tube wall 150 by low pressure chemical vapor deposition (LPCVD), wherein the pre-coating process is operated for about 14 to 16 hours at the surroundings which is about 800.degree. C. and 0.3 torr pressure. Wafer's baking process begins proceeding after the pre-coating process has been finished.
In according with the furnace purge process of above, it will result in follow problems. Referring to FIG. 2A, there are issues about remained acid after the inside tube has been cleaned, that is acid 220 will adhere to the silicon nitride coating layer 210 of the inside tube wall 200, wherein the remained acid 220 consists of water, hydrofluoric acid (HF) and OH.sup.-. The remained acid 220 will etch and damage the silicon nitride coating layer 210 at about 800.degree. C. during the pre-coating process of the inside tube is baked, as shown in FIG. 2B. If the silicon nitride coating layer 210 is etched over depth, the silicon nitride coating layer 210 of the inside tube 200 will produce slight breakages, and it will result in the silicon substrate (such as SIC--Si) of the inside tube 200 leak out, as shown in FIG. 2C. The hole 240 is formed when the silicon substrate of the inside tube wall 200 leak out for long, hence, the structure of the inside tube will be destroyed this issue, as shown in FIG. 2D.
In accordance with remained acid issues in the foregoing, two causes of the polluter for forming in the furnace process have been known: one of the particles 250 of pollution in the furnace process are chips which flakes off the silicon nitride coating layer 210 during the process; another of the particles 250 of pollution in the furnace process is silicon dioxide (SiO.sub.2) which is formed by leaking out the silicon substrate of the inside tube at high temperature. Moreover, when a pre-coating process is performed in the inside tube at high temperature, a silicon nitride layer having holes which contains nitrogen is due to nitrogen which is transported into the furnace remains in the hole which is formed by leakage of the silicon substrate, wherein the silicon nitride layer 270 which contains nitrogen is not the silicon nitride layer 210 of process. Hence, there are difference two silicon nitride layers to form, and in consequence the silicon nitride layer 210 of process is not able to coat completely on the inside tube wall, as shown in FIG. 2E. Up to now there has been no effective method in the semiconductor industry to overcome issues of foregoing conventional process.
In accordance with the above description, a method for purging furnace to decrease particle polluter in baking process is therefore necessary, so as to raise the yield and quality of the follow-up process.